Daily variations in ambient fine particulate matter (PM 2.5 ) could contribute to the morbidity of anxiety disorders in children and adolescents, but has not yet been studied longitudinally at a daily level. We tested this association using repeated weekly measures of anxiety symptom severity in a group of 23 adolescents with generalized anxiety disorder. After estimating ambient PM 2.5 concentrations using a validated model, we found that increased concentrations were significantly associated with increased anxiety symptom severity and frequency two, three, and four days later. PM 2.5 may be a novel, modifiable exposure that could inform population level interventions to decrease psychiatric morbidity.